Many practical applications exist in which it is desirable to detect pressure conditions within an enclosed environment. To this end it is desirable to use a pressure detector which avoids creating penetrations which are subject to leaks such as result when electrical leads pass into the enclosure. In such applications, a conventional approach adopted to avoid electrical penetrations has been to provide a magnetically actuated switch responsive to a magnet carried by a flexible member whose degree of deflection is determined by the existing pressure conditions. It has long been an objective of those versed in the art to find a means whereby the foregoing functions could be achieved in a simple and economical fashion. That these objectives have not been satisfactorily achieved is evidenced by the multitude of inventions proposed to this end, representative of which are those disclosed in the following patents:
______________________________________ U.S. PAT. NOS. ISSUED ______________________________________ 2,411,577 November 26, 1946 3,327,079 June 20, 1967 3,450,853 June 17, 1969 3,536,875 October 27, 1970 3,654,601 April 4, 1972 3,859,619 January 7, 1975 3,873,787 March 25, 1975 3,924,086 December 2, 1975 3,946,175 March 23, 1976 4,059,823 November 22, 1977 ______________________________________
One particular problem has especially plagued the devices of the prior art in applications in which the assembled system, including the pressure detector, is proof pressure tested in order to meet acceptance specifications. Such proof pressure testing is frequently required to test the integrity and strength of assembly welds in pressure tanks. Application of the proof pressure to the assembled system may subject the pressure detector, which is an integral part of the assembled system, to excessive pressures: a pressure of 2500 psia being typical. Any of the conventional prior art pressure responsive switches, as typified by those disclosed in the above listed patents may be expected to fail completely or at least to lose its sensitivity and/or calibration when subjected to proof pressures as high as this. The high pressure causes the resilient member, upon whose proper operation the detector relies, to exceed its elastic limit so that when the proof pressure is removed the device fails to return to its original setpoint. U.S. Pat. No. 3,982,217 attempts and partially suceeds in addressing this problem by providing a cylindrical stop means mounted inside the bellows of the device. It will be recognized that this artifice does not effectively prevent damage to the bellows under very high pressures, however, since its convolutions are not completely supported by the cylindrical stop means so that failure by way of collapse of those unsupported portions of the bellows necessarily occurs. When the elastic limit is exceeded, permanent distortion or total rupture results.